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PROGRAMMABLE HYBRID BALANCING TECHNIQUE FOR A TELEPHONE SET

Publishing Venue

Motorola

Related People

Authors:

Patrick Clément

Abstract

In a telephone set, a common way to insure that the signal generated in the microphone does not produce any signal in the earphone is achieved by taking the earphone signal between the centre points of a bridge configuration. This bridge is formed by the line impedance Z,, by an hybrid balancing irnped- ante Z,, and by resistors R, and R, according to Figure 1.

Copyright

Motorola Inc. August 1996

Country

United States

Language

English (United States)

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8 MOlVROLA Technical Developments

PROGRAMMABLE HYBRID BALANCING TECHNIQUE FOR A TELEPHONE Sl3

by Patrick CMment

In a telephone set, a common way to insure that the signal generated in the microphone does not produce any signal in the earphone is achieved by taking the earphone signal between the centre points of a bridge configuration. This bridge is formed by the line impedance Z,, by an hybrid balancing irnped- ante Z,, and by resistors R, and R, according to Figure 1.

The signal generated in the microphone causes a current to be applied on node a ofthe bridge. This will produce a line voltage vL at the 2-wire port of the telephone set. So vL usually depends on the microphone signal, on a gain factor and on both the internal impedance of the telephone set and the line impedance. When the bridge is balanced no signal is developed between the nodes b and c, the received voltage vn equals zero and thus, no signal is pro- duced in the earphone.

The simplest solution in order to balance the bridge consists in choosing R2 and 2,s such that the poles and zeros of Zu, reflect those of Z, and that R, divides Z,, in the same proportion than R, does with Z,. The drawback is here that once R,, R, and Z,, have been chosen, the balancing is real- ized for one given line impedance. When Z, changes due to a different application or due to the line length, either another set of components has to be chosen for R,, R, and Z,, or the balancing is no longer achieved.

It is assumed that, for a given line length, the hybrid balancing performance is sutlicient when the Wheatstone bridge is balanced for a 3-component model of the line. Figure 2 shows the form of Z, according to this assumption. The line impedance is modelised by an impedance presenting one pole and one zero. Then

1 +sc %iRb

bR,+Rb

1 + sCbRb (1)

z~ = (R,+Rb)

Figure 3 shows the basic idea proposed here. The receive signal is sensed on nodes a and b through components C,, R, and C,, REd. C, and C, are ac coupling capacitors. It is assumed that their value is large enough such that their effect within the voice band [300..3400Hz] can be omitted. C, and R,, are the key discrete components ofthe hybrid balancing network. Associated with R,, C, and R, produce a pole and a zero to compensate for those ofthe line model. This is achieved by means of OX42 and OTA3

connected to the reference components C, and REs in a particular continuous-time filtering configura- tion. Each of these operational transconductance amplifiers has 3 current output legs. The output legs of an CYTA may have different weighting fac- tors: 1, a, B or y in Figure 3. The ac voltage on node a, v,, is transformed into a current by means of R,, and this current is split in two parts absorbed by output legs of OTA2 and CYTA3 such that